1. Field of the Invention
This invention relates to an image forming apparatus such as a copier or a printer, and particularly to an image forming apparatus having a fixing device for fixing an unfixed image on a recording material.
2. Related Background Art
An image forming apparatus using the electrophotographic method usually has a fixing device for pinching and conveying a transfer material and a toner comprising resin, a magnetic material, a coloring material, etc. and electrostatically borne on the sheet material by the pressure contact portion (nip portion) between a fixing roller and a pressing roller brought into pressure contact with each other and being rotated and applying heat and pressure to them to thereby melt and fix the toner, and most of electric power consumed by the image forming apparatus is used by the fixing device.
In almost all of image forming apparatuses, maximum electric power usable by an entire image forming apparatus is set, and during the starting of the image forming apparatus, predetermined electric power of 70% or more of the maximum electric power is inputted to the fixing device dominating the apparatus starting time, and the other portions of the image forming apparatus are started by electric power within the remaining 30% and thus, the apparatus becomes ready for the image forming operation.
When the image forming operation is entered, the preset maximum electric power usable by the image forming apparatus, minus maximum electric power which may be consumed by the other portions than the fixing device, for example, an image forming portion, a scanner portion, a driving portion, an accessory portion, etc. at a time is maximum electric power usable by the fixing device during the image forming operation, and the value thereof is always constant.
However, if the maximum electric power usable by the fixing device when in the above-described image forming apparatus according to the prior art, a great deal of continuous printing is effected, for example, under low temperature environment is always constant, the temperature of the fixing roller will greatly fall from a target temperature due to the deficiency of electric power as shown in FIG. 3 of the accompanying drawings to thereby cause bad fixing, or in order to avoid it, the printing speed must be reduced.
For example, description will now be made by the use of a fixing device using a halogen lamp as a heating source.
The reference numeral 100 in FIG. 5 of the accompanying drawings designates a halogen heater, and a CPU 101 ON/OFF-controls electric power supplied from a power source outlet by a heater driver 104 having a switch on the basis of a temperature detected by a thermistor 103 for detecting the temperature of a fixing roller 105. The reference numeral 106 denotes a thermoswitch for preventing the excessive temperature rise of the halogen lamp, and the reference numeral 107 designates a motor driver for driving the fixing roller.
In the fixing device using the halogen lamp as described above, the electric power from the outlet is directly supplied to the halogen lamp. The heat generating amount of the halogen lamp per unit time is determined by only the ratio between the ON time and the OFF time of the switch 104 provided in an electric power supply line, and maximum suppliable electric power is constant when the ON time is 100%.
It is the merit of the fixing device using the halogen lamp that although the maximum suppliable electric power cannot be changed, it need not have a power source and therefore the cost thereof is low.
Accordingly, when as previously described, more electric power is required in such a situation as a great deal of printing under low temperature environment, for example, a fixing device like the fixing device using the halogen lamp has had only such a countermeasure as interrupting the printing or reducing the printing speed.